1. Field of the Invention
The present invention relates to a control system for vehicle windscreen defogging apparatus. More particularly, the invention relates to an automatic defogging control system for the windscreen of a vehicle in which optical means includes light emitting means for radiating a light beam toward the windscreen of a vehicle and photosensitive means for receiving the light beam reflected from the windscreen whereby when the windscreen becomes foggy due to water drops or the like sticking to the windscreen, the degree of an optical scattering caused by the foggy portion is determined from the magnitude of a photosensitive signal derived from the optical means and a defogging apparatus is operated or stopped in accordance with the result of the determination.
2. Description of the Prior Art
An automatic defogging control system of this type is known in the art by for example Japanese Unexamined Patent Publication No. 59-108939.
With this known system, when the windscreen becomes foggy so that the light beam projected to and reflected from the inner surface of the vehicle windscreen is scattered in an increased degree in other directions than in the direction of the photosensitive means, the magnitude of a photosensitive signal generated from the photosensitive means is decreased. When the photosensitive signal becomes lower than a predetermined value, the operation of the defogging apparatus is started. When the fogging is gradually eliminated by the operation of the defogging apparatus so that the degree of scattering is decreased, the magnitude of a photosensitive signal is increased and the operation of the defogging apparatus is stopped when the magnitude exceeds the predetermined value.
With the control apparatus constructed as described above, in a condition where the defogging apparatus is operated upon the occurrence of fogging and the fogging is not eliminated fully as yet, if any disturbance light such as the sunlight enters the photosensitive means, sensible frequency components of the disturbance light act to increase the magnitude of a photosensitive signal. As a result, the defogging apparatus is stopped despite the presence of the fogging. If the entry of the disturbance light is of a temporary nature, while the defogging apparatus is operated again, the fogging is not rapidly eliminated so that the stop-start operation of the defogging apparatus is repeated wastefully and the durability of the apparatus is deteriorated. If the entry of the disturbance light continues over a long period of time, the defogging effect cannot be expected at all.